they are there to assist in the mould process as it holds the top deck to the hull thus adding rigidity to the boat
then it also aids shedding the water when it washes over the gunwales- much like a self draining wet deck.
in your case i would only plug the rear one and keep the two upfront open.
when my stern tray recieves fish, i plug my rear bung holes to prevent the blood scent from attracting unwanted guests.
yes a trolley can be made to fit these holes and you may cart your boat around...
manufacturers should makes these holes bigger to fit a standard transducer... it will solve many a kayak fishermans problems.
hope this helps.

No, Faan got stuck in the garage renovating an old workbench with my son. He wants it for a desk in his flat.faan should be there now i believe